First a question: Has it ever worked??? If it worked "last year, but not now" then I'd suspect that you have "gunk" dried on the openings of the tank spray head. Often, over the winter, the black tank will dry out. If there was a "wad of tissue" that floated over the spray head during dumping/winterizing last year, that tissue may have become "cement to plug the spray head" and you'll need to fill the black tank, let it sit several days to "soften the cement" and then try to "spray it away" with water pressure through the black tank flush system. If that doesn't work,
The anti-siphon valve may be installed backwards. Essentially, it's a "one way valve" that prevents water from flowing "in reverse". So, if it's installed backwards, it would stop water from flowing into the black tank.
Typically, the anti-siphon valve is located under the bathroom sink, "way up high" nestled between the faucet connections. It'll look similar to this picture and may have blue or usually white PEX tubing connected to it.
If the anti-siphon valve is installed correctly, then the next likely cause would be a 'kink" in the PEX tubing, somewhere through the system. You might try removing the anti-siphon valve and try blowing air through from the trailer sidewall connection to the valve fitting. If you get air in that part of the system, then the kink or the plug would be from the valve to the tank. That probably would entail removing the coroplast from the trailer bottom so you can inspect/repair the plugged line.
My bet, from long distance guessing, is that you've got "gunk" dried on the spray head inside the tank.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS